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WJEC MATHEMATICS
INTERMEDIATE
FRACTIONS, DECIMALS, AND
PERCENTAGES
FRACTIONS AND
PERCENTAGES OF AMOUNTS
2
Contents
Fractions of amounts
• Non calculator
• Calculator
Percentages of amounts
• Non Calculator
• Calculator
Percentage change
• Non Calculator
• Calculator
Reverse percentage increase
One number as a percentage of another
Credits
WJEC Question bank
http://www.wjec.co.uk/question-bank/question-search.html
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Fractions of amounts - Non Calculator
To find a fraction of a number, divide that number by the
denominator and multiply the result by the numerator.
Example 1 2
7 𝑜𝑓 35
= 10
Example 2 3
5 𝑜𝑓 55
= 33
Exercise N53
Calculate the following fractions of amounts without a calculator
a. 2
5𝑜𝑓 45
b. 3
7𝑜𝑓 56
c. 4
9𝑜𝑓 63
d. 8
12𝑜𝑓 72
e. 3
4𝑜𝑓 36
f. 1
2𝑜𝑓 70
g. 5
6𝑜𝑓 36
h. 7
12𝑜𝑓 144
i. 3
13𝑜𝑓 65
Step 1
35 ÷ 7 = 5
Step 2
𝟓 ×2 = 10
Step 1
55 ÷ 5 = 11
Step 2
𝟏𝟏 ×3 = 33
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Fractions of amounts - Calculator
When you have a calculator let it do the hard work for you!
Example
5
12𝑜𝑓 156
On a calculator paper, this becomes 5
12× 156 = 65
Exercise N54
Calculate the following fractions of amounts with a calculator
a. 7
9𝑜𝑓 225
b. 3
7𝑜𝑓 175
c. 4
9𝑜𝑓 279
d. 8
12𝑜𝑓 300
e. 3
4𝑜𝑓 728
f. 1
2𝑜𝑓 449
g. 5
6𝑜𝑓 144
h. 7
12𝑜𝑓 408
i. 3
13𝑜𝑓 845
Key point!
'of' = multiply
Use the fraction button on
your calculator for this!
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Exam Questions N29
Try these questions without a calculator, then check using a
calculator
*When completing these without a calculator you may need to
practice bus stop division. See the book 'Four Operations and
BIDMAS'
1.
2.
3.
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Percentages of Amounts - Non Calculator
To find percentages of amounts we need to know the following key
facts:
Example
Find 50%, 25%, 10% and 1% of 280
𝟓𝟎% 𝑜𝑓 280 = 280 ÷ 2 = 140
𝟐𝟓% 𝑜𝑓 280 = 280 ÷ 4 = 70
𝟏𝟎% 𝑜𝑓 280 = 280 ÷ 10 = 28
𝟏% 𝑜𝑓 280 = 280 ÷ 100 = 2.8
Exercise N55
Find 50%, 25%, 10%, and 1% of the following numbers
a. 104
b. 480
c. 1000
d. 160
e. 500
f. 8
However, you may be asked to calculate a percentage other than the
four above. To calculate these, we use the above four as building
blocks
To find 50%, half the number
To find 25%, divide the number by 4 (half and half again)
To find 10%, divide the number by 10
To find 1%, divide the number by 100
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Example 1
Find 45% of 440
By breaking up 45% as seen above we can calculate the smaller
percentages and add them together
𝟐𝟓% 𝑜𝑓 440 = 440 ÷ 4 = 110
𝟏𝟎% 𝑜𝑓 440 = 440 ÷ 10 = 44
So,
45% = 25% + 10% + 10% = 110 + 44 + 44
= 198
Example 2
Find 23% of 120
𝟏𝟎% 𝑜𝑓 120 = 120 ÷ 10 = 12
𝟏% 𝑜𝑓 120 = 120 ÷ 100 = 1.2
So,
23% = 10% + 10% + 1% + 1% + 1%
= 12 + 12 + 1.2 + 1.2 + 1.2
= 27.6
45% = 25% + 10% + 10%
23% = 10% + 10% + 1% + 1% + 1%
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Exercise N56
Calculate the percentages of the following amounts
a. 35% of 150
b. 65% of 480
c. 75% of 120
d. 31% of 420
e. 81% of 500
f. 99% of 350
g. 64% of 580
h.39% of 444
i. 1.5% of 200
Percentages of amounts - Calculator
Finding percentages of amounts with a calculator requires two steps;
Example
Find 56% of 260
Becomes,
0.56 ×260 = 145.6
Exercise N57
Calculate the percentages of the following amounts
a. 26% of 189
b. 94% of 846
c. 27% of 645
d. 54% of 484
e. 68% of 1659
f. 32% of 5497
g. 24% of 8461
h.79% of 465.5
i. 31.9% of 0.54
1. Convert the percentage to decimals. (See 'Converting
between FDP' booklet for more help with this)
2. Change the 'of' to a multiply sign
As a decimal is 0.56
This is called the
multiplier
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Exam Questions N30
Try these without a calculator, then check your answers with a
calculator
1.
2.
3.
4.
5.
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Percentage Change - Without a calculator
GCSE questions often ask you to increase or decrease a number by
a percentage.
Example 1
Increase £460 by 35%
𝟏𝟎% 𝑜𝑓 460 = 460 ÷ 10 = 46
𝟓% 𝑜𝑓 460 = 46 ÷ 2 = 23
So,
35% = 10% + 10% + 10% + 5%
= 46 + 46 + 46 + 23
= 161
So, to increase, add this to the original amount
460 + 161 = £621
To increase by a percentage - add the percentage onto the
original amount
To decrease by a percentage - subtract the percentage from
the original amount
35% = 10% + 10% + 10% + 5%
Half of 10%
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Example
Decrease £460 by 35%
𝟏𝟎% 𝑜𝑓 460 = 460 ÷ 10 = 46
𝟓% 𝑜𝑓 460 = 46 ÷ 2 = 23
So,
35% = 10% + 10% + 10% + 5%
= 46 + 46 + 46 + 23
= 161
So, to decrease, subtract this from the original amount
460 − 161 = £299
Exercise N58
Increase:
a. 450 by 40%
b. 240 by 35%
c. 840 by 23%
d. 550 by 15%
e. 880 by 86%
f. 400 by 64%
Decrease:
a. 120 by 30%
b. 440 by 65%
c. 80 by 75%
d. 680 by 64%
e. 800 by 73%
f. 40 by 31%
35% = 10% + 10% + 10% + 5%
Half of 10%
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Percentage change - With a calculator
Example 1
Increase 432 by 45%
432 ×(1 + 0.45) =
432 ×1.45 = 626.4
Example 2
Decrease 954 by 31%
954 ×(1 − 0.31) =
954 ×0.69 = 658.26
Exercise N59
Increase:
a. 59 by 40%
b. 94 by 35%
c. 596 by 23%
d. 845 by 15%
e. 1652 by 86%
f. 2265 by 64%
Decrease:
a. 165 by 51%
b. 845 by 13%
c. 61 by 54%
d. 12 by 48%
e. 9451 by 54%
f. 164 by 13%
To increase by a percentage - multiply by one plus the decimal
To decrease by a percentage - multiply by one minus the
decimal
Decimal is 0.45
Decimal is 0.31 Important!
These numbers
are known as
the multipliers
13
Exam Questions N31
1.
2.
3.
4.
14
Reverse Percentage Change
Some questions will give you the price of something after the
percentage increase / decrease has been applied.
Example 1
An item in a shop has been increased by 15% and is now £460. How
much was the item to begin with
Remember, to calculate a percentage increase/decrease
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝐶𝑜𝑠𝑡 ×𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑒𝑟 = 𝑁𝑒𝑤 𝑐𝑜𝑠𝑡
So, for this example
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑐𝑜𝑠𝑡 ×1.15 = 460
We can rearrange this. (See the booklet 'Rearranging' for help with
this)
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑐𝑜𝑠𝑡 = 460
1.15
= £400
For increase, the multiplier is 1 plus the percentage as a decimal
For decrease, the multiplier is 1 minus the percentage as a decimal
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Example 2
A car costs 34% less now than it was when new. It now costs
£29700. How much was the car new?
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑐𝑜𝑠𝑡 ×𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑒𝑟 = 𝑁𝑒𝑤 𝑐𝑜𝑠𝑡
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑐𝑜𝑠𝑡 ×0.66 = 29700
Rearrange:
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑐𝑜𝑠𝑡 = 29700
0.66
= £45000
Exercise N60
1. An item in a shop was increased by 22% and sold for £549. What
was the original cost?
2. An item in a shop was increased by 78% and sold for £40.05.
What was the original cost?
3. An item in a shop was decreased by 54% and sold for £455.40.
What was the original cost?
4. An item in a shop was decreased by 19% and sold for £421.20.
What was the original cost?
This is a percentage
decrease question so
the multiplier is
1-0.34
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Exam Questions N32
1.
2.
3.
4.
5.
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One number as a percentage of another
Non Calculator
You may be asked to calculate one number as a percentage of
another.
Example
Jamie scores 21 out of 25 on a test what percentage is this?
So, we need to calculate 21 as a percentage of 25.
As a fraction
21
25=
?
100
For the denominator to change from 25 to 100 we must multiply by 4.
So we do the same to the numerator
21 ×4 = 84%
Example 2
Write 180 as a fraction of 300
Again, start with fraction and get the denominator to be 100
180
300=
?
100
To get the denominator to be 100 we need to divide by 3. We do the
same to the numerator
180 ÷ 3 = 60%
Remember: 'percent' means out of 100
Write the fraction, then get the denominator to be 100
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Example N61
1. Write 14 as a percentage of 20
2. Write 22 as a percentage of 50
3. Write 86 as a percentage of 200
4. Write 150 as a percentage of 300
5. Write 150 as a percentage of 500
With a calculator
When you have a calculator, multiply the fraction by 100 to calculate
the percentage.
Example 1
Write 45 as a fraction of 80
45
80×100 = 56.25%
Example 2
Write 47 as a percentage of 121 (to one decimal place)
47
121×100 = 38.8%
Exercise N62
1. Write 37 as a percentage of 55 (to one decimal place)
2. Write 46 as a percentage of 58 (to one decimal place)
3. Write 165 as a percentage of 241 (to one decimal place)
19
Exam Questions N33
1.
2.
